Geophysical surveys may estimate the depth, shape, and composition of subterranean formations using seismic vibrators in a Vibroseis method where seismic waves (e.g., P waves, S waves, and the like) are induced by a vibrator and then detected using one or more geophones.
A vibrator may be mounted on a truck or embodied as a large vehicle having a hydraulically powered “shaker” or actuator. The actuator may transmit vibrational energy to the earth via a baseplate coupled to the ground. More specifically, the actuator may force a mass onto the baseplate to create vibrations according to a controlled sweep of frequencies. Using “upsweep” methods, the vibrations may start at a lower frequency and end at a higher frequency. In a “downsweep” method the vibrations may start at a higher frequency and end at a lower frequency. Geophones, placed in a “spread” on or in the earth's surface, may detect seismic wave-induced vibration reflections from rock strata. Rock layers may reflect the seismic waves according to contrasting acoustic impedances between the layers. For example, an interface with a low impedance layer above a high impedance layer may reflect a large proportion of the incident wave. This reflected wave may be of greater amplitude than may be the case with an interface having layers of similar impedance. Measuring the intensity and time delay of an arriving reflected wave, at numerous locations in the spread, may allow mapping of rock strata and may provide information about the thickness and composition of earth layers.
The vibrator may be located within the geophone spread and may, however, produce source generated noise that masks wave reflections recorded by the geophones.
Source generated noise may include, for example, air wave noise. Specifically, since the vibrator is exposed to air, some of the vibrator's vibrational energy (e.g., from an upper surface of the baseplate) may be transmitted through the air as sound waves. The noise may be strongest at higher frequencies (e.g., 30 Hz and above). The air wave noise may include coherent noise, which may have a consistent phase from trace to trace. Air waves may be coupled to air and, in the case of low frequency waves (e.g., 6-8 Hz), partially coupled to near surface waves. A surface wave may include a wave (e.g., Love and Rayleigh waves) that propagates at the interface between two media (e.g., earth surface and air) and may result in, for example, ground roll. Ground roll is a type of coherent noise with low-velocity, low-frequency, and high-amplitude characteristics.
Source generated noise may also come from, for example, the vibrator's engine and movement of other mechanical components coupled to the vibrator. The vibrator's engine may provide power to move the vibrator from one shot site to another and may also supply power to hydraulic pumps. The pumps may generate pressure for hydraulic accumulators that store power used to operate the actuator (e.g., raise/lower the actuator to/from the ground and perform sweeps with the actuator). Also, harmonics caused by, for example, the non-linearity of the vibrator's servo-valve and base plate flexure can result in unwanted noise. Additional noise can be generated by other vibrators sweeping in the vicinity (e.g., during Simultaneous Source Acquisition) or from vehicular traffic such as another vibrator moving to another shot point.
Methods have been suggested to counter various types of noise (e.g., air wave noise) by making modifications to the vibrator itself or using active cancellation. However, certain types of noise (e.g., noise from vibrator engine, vibrator cooling system, near-by vibrators, nearby vehicular traffic) have been difficult to address.